This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Clathrin-mediated endocytosis is a fundamental process in living cells, allowing them to capture macromolecules efficiently from the extracellular environment and package them into vesicles in the cytosol. Numerous diseases can be traced to a defect in the succession of molecular reactions leading to endocytosis. In the budding yeast Saccharomyces cerevisiae, endocytosis is localized to cortical actin patches, where a dense network of actin filaments is polymerized around clathrin-coated pits. This network is regulated by a complex set of proteins which cooperate in order to generate membrane invagination. We have recently developed in our laboratory a system in yeast extracts for the assembly of actin based structures around polystyrene microbeads. We believe that the protein composition of these structures could be similar to the protein composition found in vivo in cortical actin patches. We isolated the beads with their associated actin structures, and we would like to determine the protein composition by using mass spectroscopy. These results should allow us to further analyze the role of the different components during endocytosis.